1. Field
The present disclosure relates to electric motors, more specifically to control methods for high speed motors.
2. Description of Related Art
When a brushless 3-phase DC motor is required to meet both low speed and high speed operating points, the motor is typically designed so that the required low speed torque is achieved with the allowable amount of phase current. This leads to a light weight motor but also a relatively high motor back emf (bemf) constant which may prevent the motor from being able to meet the high speed requirement.
A common solution to this problem is to introduce a field weakening component on the direct axis (d-axis) to the motor phase current during high speed operation. Direct axis current (Id) counteracts the bemf caused by the rotor magnets, thus allowing the motor to spin faster at a given level of supply voltage and load. The Id current creates heat in the motor but does not produce useful torque. This approach effectively reduces the motor bemf voltage so that more net voltage is available to drive quadrature axis (q-axis) current (Iq) which produces useful motor torque.
The field weakening current can be either controlled directly, as in the case of field oriented control (FOC), or indirectly by controlling the rotor phase advance. Phase advance is the angular amount by which the motor current is switched on ahead of where it normally would be based on motor shaft position, which can be calculated as arctan(Id/Iq). Phase advance beyond 90 degrees can cause torque reversal and associated unstable operation.
FOC is a motor current control method that uses the Clarke mathematical transform to convert the three measurable motor phase currents into theoretical, direct, and quadrature axis currents. The transform is based around the angle of the motor shaft, so during operation at constant speed and load, the resulting Id and Iq currents are constant, even though the three phase currents are alternating at high frequency. This characteristic makes the FOC method useful for directly controlling the Id and Iq components of motor phase current.
Traditional methods for specifying Id use a table with speed as the independent variable. Usually with this method Id is not used until motor speed has increased beyond the low speed operating point. Typically Id is then increased linearly up to a maximum value which is attained at a speed less than the high speed design point.
The open loop nature of this control has the drawback that it introduces an unstable (positive feedback) character into the speed control. For example, as speed increases, Id increases, which leads to additional speed, etc. Also, for loads less than the design point, this method will maintain the design point level of Id current, which will be more than is needed, thus leading to higher phase current, more motor heating, lower efficiency, and excessive phase advance. Additionally, for loads greater than the design point, this method will maintain the design point level of Id current, which will be less than is needed, thus leading to speed droop. If speed droops far enough, the method will encounter the low speed to high speed transition portion of the table which will cause speed to increasingly droop with positive feedback.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for more efficient motor control systems and methods. The present disclosure provides a solution for this need.